This invention concerns electronically readable identification tags for inventory control, automated sales, tracking shipped items, theft detection, counterfeit detection and other purposes, and more specifically the invention concerns particular embodiments of and enhancements to the ID tag device and reader system disclosed in U.S. Pat. No. 6,480,699. The invention provides for high efficiency in the receiving and storage of power in the tag device and in transmitting a signal from the tag essentially without interference from the reader signal.
A number of identification tags have previously been proposed and used to electronically control, detect and track a variety of items to which these tags are attached. These are usually in the form of a card including an antenna. When affixed to or embedded into virtually any object, the tag can individually identify the object using a unique, factory-programmed unalterable code held in a memory mounted on the card and connected to a transponder. The transponder in such a card or tag can reflect an incident signal from a reader unit in real time as described, for example, in U.S. Pat. No. 5,479,172. However, such reflective transponders in this type of identification tag require transmission of a large amount of energy from the reader, especially if the reader is to function at any practical distance, such as several meters. The reflected signal received by such a conventional reader will have a strength far smaller than the strength of the signal sent out by the reader.
U.S. Pat. No. 6,480,699, which is hereby incorporated into this disclosure by reference, describes an electronic identification tag that actually captures and stores energy received from the incident signal sent by a reader, then uses that energy to enable transmission of a responsive signal. The radio frequency (RF) signal sent by the reader, which may be referred to as a power signal, contains energy of which a portion is absorbed and temporarily stored in an energy storage device in the tag. This stored energy is then used to support circuitry in the tag and to generate a reply signal of sufficient strength as to be picked up by the reader several meters away, as well as at much shorter ranges. The reply signal is encoded by the tag device with data from a data source on the tag, primarily including an identifying code for the particular item, a code which is unique among all similar items. The antenna for receiving the power signal and also for transmitting the reply or data signal (preferably the same antenna) is directly on the integrated circuit in the '699 patent, rather than being removed from but connected to the IC. The tags can be extremely small, less than 1 mm in thickness, preferably less than 0.5 mm thick, each side preferably less than 2 mm. Such an antenna is, therefore, potentially extremely small and possesses wide application.
The IC or chip of the '699 patent therefore included the antenna, the power supply, a radio receiver, a radio transmitter, a digital logic circuit and a memory block. The preferred tag structure of the '699 patent was devoid of conventional IC parts such as bond pads, bond wires, lead frame or package terminals (such as leads of a pin grid array package or pads of a ball grid array package).
In such an electronic transponder identification tag as described in the '699 patent, power management is a challenge. The quantity of energy captured by the power supply of each of the multitude of ID tags in the system, from the power signal sent out by the reader, must be sufficient to enable a strong enough data signal to be sent out by the tag, for reception by the reader which might be several meters away. The power can be gained incrementally by the power supply of the tag, over a period of time, but this time cannot be excessive or the system of readers/tags will not work effectively for many situations. If the device is extremely small, such as to be embedded in thin items which might include paper money or other papers, the antenna is so small as to create further problems of power management, in that the antenna must both capture RF energy and transmit a sufficient signal to be received. Also, the more electronic components that must be powered on the IC of the tag, the greater the power requirement, and thus further complications in power management. For efficient power management, componentry on the chip should be kept to a minimum.
If the tag is required reliably to produce an outgoing data carrier frequency within close tolerances, this can require some power-consuming components onboard the chip, and thus it would be desirable to avoid such a requirement, and this is one issue addressed by the present invention. Further, the potential for interference between the reader signal and the tag transmission must be avoided or minimized, preferably without adding power-consuming components.
Another issue relates to storing sufficient voltage in the tag's power supply for the tag to operate. The small size of the tag and the small onboard antenna make this difficult in some circumstances, for a system which operates within FCC regulations. A minimum voltage threshold must be met for the tag to respond.
A further issue can be voltage regulation at the tag to protect transistors onboard the chip from harmful high voltages that can be induced on the antenna. Circuitry connected to the antenna should be capable of regulating the desired voltage to the power supply circuit.
Still another issue for the contemplated system is the potential for interference with other communications devices nearby. To operate the system of the invention, a minimal amount of power should be used to accomplish the desired result.
In addition to all these concerns, such a tag if manufactured in extremely small size and using a high volume semiconductor process, can encounter variations in manufacturing, shifting the center frequency from tag to tag based on the values of the inductor and capacitor on the tag. Power storage is adversely affected, as is the ability to transmit the response.